How to Calculate Anhydrous and As-Is Assay by HPLC
First of all understand that anhydrous assay and as-is assay are two different ways of expressing the same potency depending on whether water/moisture is excluded.
1. Basic concept
If an API contains water, the as-is sample weight includes the water, while the anhydrous basis excludes the water.
The relationship is:
As-is assay (%) = Anhydrous assay (%) × (100 − Water content %)/100
And:
Anhydrous assay (%) = As-is assay (%) × 100/(100 − Water content %)
2. Example: Anhydrous assay → As-is assay
Suppose your HPLC result is:
Anhydrous assay = 100.2%
Water content = 0.45%
First calculate the dry/anhydrous fraction:
100−0.45=99.55%100 – 0.45 = 99.55\%
Therefore:
As−is Assay=100.2×99.55100As-is\ Assay = 100.2 \times \frac{99.55}{100} =99.7491%= 99.7491\%
Result
Anhydrous assay = 100.20%
Water content = 0.45%
As-is assay = 99.75%
So, if the specification is 95.0–105.0% on an as-is basis, the result would be 99.75%.
3. Reverse calculation: As-is → Anhydrous
Suppose:
As-is assay = 99.75%
Water = 0.45%
Then:
Anhydrous Assay=99.75×10099.55Anhydrous\ Assay = 99.75 \times \frac{100}{99.55} =100.20%= 100.20\%
So the two results correspond to each other:
100.20% anhydrous ≈ 99.75% as-is
4. Why is the as-is result lower?
Imagine you weigh 100 mg of material.
If water content is 0.45%, then:
Total sample = 100 mg
Water = 0.45 mg
Anhydrous material = 99.55 mg
Therefore, the actual drug substance available on an as-is basis is diluted slightly by the water.
That is why:
Anhydrous assay > As-is assay
when moisture is present.
5. Important point for HPLC calculation
In pharmaceutical analysis, you should first check how the analytical method and specification define the assay.
For example, if the specification says: Assay: 98.0–102.0%, calculated on an anhydrous basis
then your final result needs to be corrected to the anhydrous basis.
If it says: Assay: 98.0–102.0%, as-is basis
then you should report the as-is result.
Also, don’t automatically correct an HPLC result for water just because KF was performed—the calculation should follow the monograph, specification, and approved analytical procedure.